1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image from an input video signal, and more particularly to such apparatus particularly adapted for use in a television system utilizing 625 horizontal scanning lines.
2. Description of the Prior Art
The so-called NTSC television system has 525 horizontal scanning lines per frame, while the CCIR television systems such as a PAL, SFCAM etc. generally have 625 horizontal scanning lines per frame. The effective image frame generally contains about 480 horizontal scanning lines in the case of the NTSC system, or about 576 lines in the case of CCIR systems. The sampling of the television signal for image recording is generally conducted in the vertical direction of the frame, since sampling along the horizontal direction requires a very high sampling frequency and accordingly a high-speed A/D converter. The image recording of a line is conducted by the data thus sampled in the vertical direction, and that of a frame is achieved by horizontally displacing, in succession, the position of said vertical sampling.
A vertical sampling of the NTSC signal provides about 480 samples per effective frame, while that of the CCIR signal provides about 576 samples, thus giving rise to a different image print size. Consequently, printers for the CCIR system have been larger than that for the NTSC system, and it has been necessary to develop completely different printers for the NTSC system and for the CCIR system.